The purpose of the proposed research is to investigate the relationships between morphological and physiological characteristics of vestibular nuclei and primary afferent neurons. Electrophysiological studies will be conducted on decerebrated gerbils to examine the responses of neurons in the major vestibular nuclei and selected accessory nuclei to time-varying linear head acceleration and to angular acceleration about the vertical head axis. Linear force will be applied in the form of sinusoidal translational head acceleration at single fundamental frequencies ranging, from 0.2 to 5.0 Hz, along the cardinal horizontal and vertical head axes. This study will provide data related to mechanisms for convergence of input from semicircular canals and otolith organ receptors onto single neurons of the vestibular nuclei. In addition, measures of the relationship between the stimulus vector and the nature of the neuronal response will be used to test hypotheses concerning the functional organization of the central otolith related system. Two sets of anatomical experiments will be conducted. Vestibular nuclei neurons that exhibit responses to linear head acceleration will be injected intracellularly with horseradish peroxidase (HRP) to label the cell bodies and their processes. Physiological characteristics and morphological features will be compared. In double labeling experiments, physiologically identified otolith-related afferents will be intra-axonally injected with HRP; concurrently, ipsilateral vestibular nuclei neurons that respond both monosynaptically to electrical stimulation of the labyrinth and to linear forces will be labeled intracellularly with a fluorescent dye, e.g. Lucifer Yellow or Texas Red. This will provide direct anatomical identification of second order neurons and both physiological and anatomical description of their primary vestibular input from otolith related afferents.